Transmitting high power signals using wireless transceivers can be challenging due to the inherent non-linear characteristics of power amplifiers (PAs) provided within the transceivers. Indeed, power amplifiers typically have a limited linear region of operation for which their operating characteristics are predictable. For example, if the operating temperature and/or power level of a power amplifier falls outside a specified range, the power amplifier may begin operating in a non-linear manner that undesirably distorts the data signal being amplified for subsequent transmission by an antenna to a receiving device. The resultant signal distortion can lead to data errors in the receiving device, and can overload and even damage front-end circuitry in the transmitting device. To compensate for the inherent non-linear characteristics of the power amplifier, pre-distortion techniques can be applied to maintain the signal power levels within certain ranges (e.g., by adjusting the gain of the power amplifier).
More specifically, many transceivers include a loopback path that routes the signal output from the power amplifier to a pre-distortion circuit, which in turn processes the loopback signal to calculate a set of values that represents the behavior of the power amplifier (e.g., by comparing the loopback signal with the original signal). These values can be used to calibrate the power amplifier, for example, by pre-conditioning the signal before it is applied to the power amplifier and/or adjusting one or more settings (e.g., the gain setting) of the power amplifier. For many wireless communication devices that employ such calibration techniques, the loopback signal is routed to the pre-distortion circuit via the device's receiver circuitry to avoid an unnecessary duplication of device resources. To achieve proper calibration, the loopback signal should be very accurate (e.g., having an error of less than approximately −40 dBm).
Many multiple-input multiple-output (MIMO) wireless communication devices employ multiple transmit and receive chains and antennas to increase transceiver data rates and/or to achieve signal diversity in communication channels affected by multipath fading. Unfortunately, MIMO transceivers are prone to performance degradations (such as inter-symbol interference caused by multipath dispersion and inter-channel interference or cross-talk) because of limited isolation between the transmit chains when they are concurrently transmitting signals. More specifically, because the isolation between transmit chains in many MIMO wireless devices is approximately 20 dBm, during calibration of a first transceiver chain in a wireless device having two transceiver chains, the second transceiver chain is typically disabled (e.g., precluded from transmitting signals) to prevent signals transmitted by the second transceiver chain from being inadvertently received by the first transceiver chain and interfering with the loopback signal associated with calibrating the first transceiver chain. Indeed, many MIMO wireless devices reduce transmit operations to a single chain (i.e., the calibrating chain) in a round-robin scheme for all transmit chains until the calibration process is complete, thereby effectively reducing the device to single chain transmission, which undesirably degrades transmission data rates.
Thus, for MIMO wireless transceivers, it would be desirable to allow one transceiver chain to be calibrated while the other transceiver chain is concurrently transmitting data.